Not a junco

Here's a riddle:  When is a junco not a junco?  Answer:  Why, when it's a yellow-rumped warbler, of course.   But damned if I wouldn't have sworn up and down that this was a junco when I was watching it.  It was feeding on the ground like a junco, and to the naked eye at about 30 feet, it looked exactly like a junco --dark above and light below.  And besides, there were other juncos around at the time.  But you know what they say about the camera -- it adds 30 pounds, and turns juncos into yellow-rumped warblers.

I later found out from the paper that this fall yellow-rumped warblers had been seen by watchers in enormous numbers -- flocks of over a thousand.  Strange, that.  I guess they can spare one for Somerville.